1. Technical Field
The present invention relates to surface light source devices and liquid crystal display devices, and more specifically, to surface light source devices for use as backlights for liquid crystal display devices and to the liquid crystal display devices.
2. Related Art
With the reduction in weight and thickness of devices such as mobile phones, the liquid crystal display devices for use in such mobile phones are desired to have lighter weights and reduced thicknesses. The surface light source devices for use as backlights for the liquid crystal display devices are desired to have even more reduced thicknesses accordingly.
An exemplary surface light source device with a reduced thickness is disclosed in JP2009-054327A (a planar light unit). A cross section of a liquid crystal display device using the surface light source device is illustrated in FIG. 1.
In a surface light source device 11 described in JP2009-054327A, a reflector sheet 12, a film-like light guide plate 13, a diffuser sheet 14, and two prism sheets 15a and 15b are set in a frame 16 having a shape similar to a picture frame. The reflector sheet 12 is applied to the lower surface of the frame 16 by means of a reflective sheet tape (not shown), while a rim sheet 17 is applied to the upper surface of the frame 16. Further, the frame 16 is applied to the outer peripheral surfaces of the light guide plate 13 by means of a double-sided tape (not shown). An end portion of the light guide plate 13 projects from edges of, e.g., the reflector sheet 12 and the diffuser sheet 14, and light sources 19 are fitted in openings 18 formed in the end portion. A liquid crystal panel 20 is laid on the surface light source device 11 at a position in opposition to the prism sheet 15b. In this fashion, a liquid crystal display device 21 is configured.
The surface light source device 11 uses a film-like light guide plate 13 and a sheet-like frame 16 that is formed from, e.g., a PET sheet or a SUS sheet punched by a press, thereby achieving reduction in overall thickness of the surface light source device 11.
In the surface light source device 11 of JP2009-054327A, the frame 16 is applied to the outer peripheral surfaces of the light guide plate 13 by means of a double-sided tape, and thereafter the reflector sheet 12 is applied to the lower surface of the frame 16 by means of a reflective sheet tape. The rim sheet 17 with the prism sheet 15b, the prism sheet 15a, and the diffuser sheet 14 applied to the lower surface thereof in order is applied to the upper surface of the frame 16, so as to set the diffuser sheet 14 and the prism sheets 15a and 15b within the frame 16.
Since the light guide plate 13 is merely applied at its outer peripheral surfaces to the frame 16 by using a double-sided tape, the light guide plate 13 is not located heightwise (along the thickness) at a satisfactory level of accuracy relative to the frame 16. For this reason, the light guide plate 13 may tilt within the frame 16, causing inclination from the vertical direction in the direction of the light emitted from the surface light source device.
Also, since the light sources 19 are merely inserted within the openings 18 in the light guide plate 13, the light sources 19 are not fixed sufficiently firmly, resulting in unsatisfactorily accurate locating of the light sources 19 along the thickness of the light guide plate 13.
Further, since the light guide plate 13 is applied at its outer peripheral surfaces to the frame 16 by means of a double-sided tape, the difference in indices of refraction is small at the outer peripheral surfaces of the light guide plate 13. Thus, the light that has been guided through the light guide plate 13 to reach the outer peripheral surfaces leaks out from the outer peripheral surfaces due to the adhesive of the double-sided tape, which makes a loss. This constitutes one of the causal factors in lowering of brightness of the surface light source device.
Generally, even a thin light source (an LED) has a thickness on the order of 400 μm (the height of the light outgoing window is on the order of 300 μm). In case of using a planar, thin light guide plate 13 (125 μm), it is difficult to guide the light emitted from the light sources 19 efficiently into the light guide plate 13. The light use efficiency is thus decreased. Even if a light source that is as thin as the light guide plate 13 exists, such a thin light source presumably gives off light of a small quantity, which leads to insufficient brightness of the surface light source device and may not be suitable for practical use.